Sounds like a bacteria to me. You need to take him to the doctor to have this looked at and get him on some antibiotics. He probably needs a tetanus shot as well.
it sounds like it's infected. take him to a doctor to have it checked out immediaetly
maybe an urgent care doc can take a look and clean the wound. you'd hate your kid to lose his foot due to untreated infection...if no way to get to doctor, clean the wound with saline or diluted hydrogen peroxide a few times a day, apply some neosporin ointment, cover the wound, elevate it if it throbs, make sure they get good sleep and diet. if red lines move up leg from wound or if he runs a fever, take to doc immediately due to risk of worse infection